- a change to the previous __mapper_args__ commit.

- the __mapper_args__ dict is copied when propagating to a subclass,
and is taken straight off the class __dict__ to avoid any
propagation from the parent.  mapper inheritance already
propagates the things you want from the parent mapper.
[ticket:1393]

CHANGES

@@ -166,9 +166,11 @@ CHANGES
     in theory make it easier for custom metaclasses to modify
     the state passed into _as_declarative.
 
-   - the __mapper_args__ dict is copied when propagating to a subclass.
-     Still need to decide how the argument propagation should
-     work in the bigger picture.  [ticket:1393]
+   - the __mapper_args__ dict is copied when propagating to a subclass,
+     and is taken straight off the class __dict__ to avoid any
+     propagation from the parent.  mapper inheritance already
+     propagates the things you want from the parent mapper.
+     [ticket:1393]
      
 - mysql
   - Fixed reflection bug whereby when COLLATE was present, 

lib/sqlalchemy/ext/declarative.py

@@ -499,8 +499,11 @@ def _as_declarative(cls, classname, dict_):
                 if not table.c.contains_column(c):
                     raise exceptions.ArgumentError(
                                 "Can't add additional column %r when specifying __table__" % key)
-            
-    mapper_args = dict(getattr(cls, '__mapper_args__', {}))
+    
+    if '__mapper_args__' in dict_:
+        mapper_args = dict(dict_['__mapper_args__'])
+    else:
+        mapper_args = {}
     if 'inherits' not in mapper_args:
         for c in cls.__bases__:
             if _is_mapped_class(c):

test/ext/test_declarative.py

@@ -924,13 +924,15 @@ class DeclarativeInheritanceTest(DeclarativeTestBase):
             __tablename__ = 'people'
             id = Column(Integer, primary_key=True)
             discriminator = Column('type', String(50))
-            __mapper_args__ = {'polymorphic_on': discriminator}
+            __mapper_args__ = {'polymorphic_on': discriminator,'polymorphic_identity':'person'}
             
         class Engineer(Person):
             primary_language = Column(String(50))
 
         assert 'inherits' not in Person.__mapper_args__
-    
+        assert class_mapper(Engineer).polymorphic_on is None
+        
+        
     def test_custom_join_condition(self):
         class Foo(Base):
             __tablename__ = 'foo'